First off, 63 grains of IMR4350 under a 150-grain bullet in a .300 WSM case seems to be about 6 or 7 grains below max in several sites giving loads for it. A 10% reduction in charge weight below max often doesn't produce enough peak pressure to push the back end of the case against the bolt face after it's driven hard into the shoulder by the firing pin. That'll end up with the primer sticking out of the case head the amount of head clearance between the case head and bolt face.

Second, if Hart says the chamber headspace is .004" over or long, that probably means it's that much greater than the SAAMI spec of 1.726" for the GO gauge. As the spec for the NO GO gauge is 1.736: that rifle's headspace is right in the middle of the normal and safe range.

Third, no centerfire rifle shooter winning matches and setting records wants any bolt binding whatsoever when the round's chamberd. Even the slightest amount, especially on actions without the bolt face squared and locking lugs lapped to full contact, will cause the bolt to close at different places. That ends up causing inconsistant forces transferred to the barreled action for each shot making the barrel whip differently and the bullets don't all leave in the same direction. I know the "a slight bind" of the bolt on a chambered round is often believed to more firmly and accurately position the round in the chamber, but it's another myth in the shooting sports. If you shoot your stuff well enough to see the difference, you'll agree with me on this.

Fourth and final.... best accuracy and case life typically happens with fired rimless bottleneck cases that are full length sized and their shoulder set back no more than a couple thousandths. You'll need a case headspace gauge (RCBS Precision Mic or equal) to measure your fired and sized cases to see how much setback the shoulder gets. Full length bushing dies from Redding or RCBS are those in vouge these days. Or just lap the neck out of a standard full length sizing die to about 2 thousandths smaller than a loaded round's neck diameter like folks did decades ago before bushing dies were available.

I'm assuming you were refering to the question I asked about measuring the primer pockets. Honestly, you can get a rough idea to what you have with a pair of calipers, but kinda expect the inside diameter measurment to be off a thousandth or so, but still close enough right now. You can measure the depth off the base of the caliper, and simply measure the I.D. like you would the neck I.D. I don't remember off the top of my head what they spec is for the interference fit between the primer and the I.D. should be, but I suspect a thousandth and a half to be about minimum. The depth sould be slightly deeper than the higth of a new unfired primer. I like my primers to be about four to five thousandths under the face when seated. Anything shallower, and your asking for trouble.

Did you seat your primers by a hand tool or simply did it off the press your using? You really can get a better feel with a hand priming device, but some presses do seat primers very accurately. Also while on the subject of primer pockets, it's common for primer pockets to actually enlarge under high pressure firing. Just the nature of the beast. I have just one manual with the loading data for the cartridge, and they spec a Winchester mag primer. I prefer Federals myself. They are harder and seem to go in tighter than the others out of my primer stash. I use almost nothing but Federals and CCI's with a spattering of the others here and there (I do like the Remington Benchrest primers whenever I can find some). A change of brand in the primers might be worth the effort and a hundred primers are cheap to experiment with.
gary